Acute myocardial infarction with left ventricular dysfunction and those with heart failure are major clinical entities with an extraordinary impact in our society. Administration of drugs (diuretics, angiotensin converting enzyme inhibitors, angiotensin receptor blockers, calcium channel antagonists, etc.) for treating these conditions is not without side-effects and many of the drugs are extremely expensive.
Most of the therapeutic effects of the above drugs are combined in the natriuretic peptides ANF (atrial natriuretic factor) and BNP (brain natriuretic peptide). These are polypeptide hormones produced by the heart that share potent vasodilatory, diuretic, natriuretic and antigrowth properties. They also inhibit renin and aldosterone production and sympathetic tone. In addition, these hormones exert their effects while simultaneously preventing rebound of the renin-angiotensin-aldosterone and sympathetic systems. The treatment of decompensated heart failure with ANF has been carried out with promising results. In contrast, there is some controversy regarding the therapeutic use of BNP (Sackner-Bernstein et al. Short-term risk of death after treatment with nesiritide for decompensated heart failure: a pooled analysis of randomized controlled trials. JAMA. 2005; 293:1900-1905).
The biological effects exerted by ANF and BNP are mediated, in humans, by the circulating peptides ANF[99-126] and BNP[77-108] through the membrane-bound guanylyl cyclase receptor A (NPR-A) thereby increasing intracellular 3′,5-cyclic guanosine monophosphate (cGMP).1 cGMP plasma levels and urinary excretion increase in parallel to increases in ANF and BNP plasma concentrations and hence, determination of cGMP in plasma or its excretion in urine can be used as a reflection of ANF or BNP biological activity. BNP is about 10 fold less potent than ANF in promoting cGMP production. ANF and BNP are metabolically cleared by the NPR-C receptor and by neutral endopeptidase, which is present most notably in the kidney.
ANF has anti-fibrotic properties and prevents hypertrophy of cardiocytes through a process that involves the activation of NPR-A.
Comparative binding studies to NPR-A have shown that Kd is markedly less for BNP compared to ANF (Kuhn M. Molecular physiology of natriuretic peptide signalling. Basic Res Cardiol. 2004; 99:76-82). From a therapeutic point of view therefore, the use of ANF appears more advantageous than the use of BNP. In addition, the amino acid sequence of ANF is highly conserved, which facilitates the use of non-human test systems (see Kuhn, 2004, supra.).
The peptide nature of ANF prevents its administration by ingestion and even when injected, ANF has a very short (approximately 2 min) half-life in blood due to its rapid clearance. The need for continuous intravenous (iv) infusion limits its use to a hospital setting. Coupling of ANF to human serum albumin (HSA) using a chemical maleimide linker resulted in a chemically cross-linked fusion protein that was shown to have greater stability compared to ANF alone in an in vitro human blood plasma assay (Leger R, Robitaille M, Quraishi O et al., Bioorg Med Chem Lett. 2003; 13:3571-3575). However, chemical cross-linking of ANF to serum albumins could generate errors innate to the cross-linking procedure raising important limitations concerning the exact formulation and reliability of such pharmaceutical preparations, including immunogenicity. In addition, the cost of production of ANF-HSA by chemical means is commercially prohibitive.
Therefore, there is a need for an ANF analogue with significantly longer half-life in a body fluid, for example, blood.